The importance of automatic identification systems increases particularly in the service sector, in the field of logistics, in the field of commerce and in the field of industrial production. Further applications of identification systems are related to the identification of persons and animals.
In particular contactless identification systems like transponder systems (for instance using an RFID tag) are suitable for a wireless transmission of data in a fast manner and without cable connections that may be disturbing. Such systems use the emission and reflection/absorption of electromagnetic waves, particularly in the high frequency domain.
A shortcoming of the RFID technology is that undesired cross-talk may occur when multiple tags are to be read by multiple gates or readers.
U.S. Pat. No. 6,943,725 discloses an access control system for an object, particularly a motor vehicle, which comprises at least one base station with a transceiver device that transmits a wideband modulated interrogation signal, at least one transponder that has a modulation device, in order to modulate an auxiliary carrier signal, the frequency of which is changed between an infinite number of frequency positions, onto an interrogation signal received at a distance from the base station and reflect it as a code-modulated response signal, and an evaluation device that is connected after the transceiver device and that evaluates the response signal in sidebands of the changed frequency positions, with respect to code and distance.